Infrastructure equipment for the storage of large quantities of liquid is required in a very wide variety of industries, for example in the petroleum industry, the chemical industry and also the food-and-drink industry. The storage capacity of these liquid-storage systems is by way of example 50 000 cubic meters, but a very wide variety of sizes can be provided, depending on each user's requirement. Flexible containers are widely used, by way of example made of plastic or rubber and also termed pillow tanks or flexi-tanks, these being filled with liquid and used for storage by being placed simply on the ground or, to relieve pressure, between embankments. Alternatively, the filled flexible containers can be stored in water basins for protection from mechanical damage caused by the surroundings and to relieve pressure on the flexible containers, as described by way of example in WO-A1 2008/014203. The flexible containers of WO-A1 2008/014203 are of cylindrical design, with flexible side walls and flattened, rigid or stiff bases and, respectively, tops. U.S. Pat. No. 2,798,639 also describes comparable systems. WO-A1 81/03011 discloses spherical flexible containers. In comparison with metal tanks, flexible containers are lighter and moreover foldable, and therefore amenable to faster and simpler installation, and also not susceptible to rusting.
A further challenge consists in finding a simple and efficient method that allows maximum completeness of discharge, from the flexible containers, of liquids located as stored product in these flexible containers, thus also reducing the cost of cleaning of the flexible containers after discharge of the liquids. It is moreover very difficult or indeed impossible to achieve reliable determination of the quantity of liquid located in the flexible containers known hitherto.